The invention relates to a hydraulic dual circuit brake system for road vehicles, and particularly to an anti-locking brake system that operates with hydraulic power assistance.
Anti-locking brake systems of this type are required in particular for heavy and powerful passenger vehicles in the upper power and comfort range in order to be able to achieve the high braking decelerations needed for safe driving. According to the statutory regulations, they must further be designed such that, minimum braking of the vehicle of 0.3 is still achievable with a pedal force of maximum 500N in the event of a failure of the power assistance. Furthermore, it is also necessary for safety reasons that the reaction forces occurring on the brake pedal provide the driver with a reliable response indicating the functional state of the brake system, whether it is in normal operation or whether it is with the anti-locking brake system (ABS) control active, and thereby signals to the driver that a braking operation is taking place in a situation of potential danger.
A brake system of the type stated at the start is known from German Unexamined Published Patent Application No. 3,110,028. In this known brake system, the brake circuit assigned to the primary output pressure chamber of a tandem master cylinder is dynamically operated (i.e. it admits the output pressure of the auxiliary pressure source) and the other brake circuit is connected to the secondary output pressure chamber of the tandem master cylinder which chamber is bounded by a floating spool and is statically operated. The floating spool is impinged by the output pressure of the auxiliary pressure source which is coupled into the primary output pressure chamber of the tandem master cylinder. The primary piston of the tandem master cylinder is impinged from both sides by the output pressure of the auxiliary pressure source and can therefore also be displaced without reaction force by the brake pedal as long as the auxiliary pressure source does not fail. In the event of a failure of the auxiliary pressure source, the tandem master cylinder operates like a conventional pedal-operated master brake cylinder. In normal operation (i.e. with operative auxiliary pressure source) the important response for the driver, indicating the brake force introduced, is achieved by means of a displacement simulator. The displacement simulator couples an output pressure proportional to the pedal force into the dynamically controlled brake circuit for controlling the proportional control valve.
One of the disadvantages of this known brake system is that, although the pedal restoring force generated by the displacement simulator is correlated with a certain pedal travel, it is not, however, correlated with the quantity of brake fluid actually taken up by the wheel brakes. Such a quantity of brake-fluid can be affected differently depending on the degree of bleeding of the brakes and/or as a consequence of a vapour lock. Another disadvantage is that if the pressure source fails (i.e. if both brake circuits are operated statically) high pedal forces are required in order to achieve an adequate brake deceleration.
Although these disadvantages have been reduced in a further known power-assisted brake system of a similar type (SAE Technical Paper Series 830483, March 1983) to such an extent that, in this brake system, if the auxiliary pressure source fails, a mechanical pedal force transmission becomes effective, which in this system favors the brake force development in the front axle brake circuit. A failure of the auxiliary pressure source is, however, always associated with a failure of the dynamically operated rear axle brake circuit, with the consequence that the maximum braking effect physically achievable is markedly reduced.
One object of the present invention is therefore to create a brake system of the type stated at the start which provides the driver with reliable information on the functional state of the brake system via the reactional behavior of the brake pedal and, in the statistically significant cases of malfunction, in particular in the event of a failure of the auxiliary pressure source, make it possible to achieve a high braking deceleration at favorably low pedal forces.
Hereafter, as long as the ABS control is not in operation, both brake circuits are statically operated and the reaction behavior of the brake pedal is the same as in the case of direct pedal actuation of the primary piston of a tandem master cylinder. The present invention provides transmission of the pedal force in the event of a failure of the power assistance caused by different values of the effective cross-section of the primary piston with or without power assistance. A clear transmission of the pedal force becomes effective since the pressure build-up in the primary output pressure chamber of the tandem master cylinder is brought about by displacement of a plunger in the present invention. This plunger is designed as a part of the primary piston, the cross-section of which is less than that of the primary piston effective in power assistance. Although there is an extension to the pedal travel if the auxiliary pressure source fails since the maximum available displacement stroke of the primary piston in a failure of the auxiliary pressure source reaches from the basic position to the striking position of the primary piston against the secondary piston, which for its part runs up against the end face of the cylinder casing. High brake pressures and correspondingly high brake decelerations far in excess of the statutory minimum requirements can none the less be achieved in the event of a failure of the auxiliary pressure source. In the case of activation of the ABS control, the brake pedal becomes as it were "hard," so that the driver also obtains a clear response via the brake pedal for this operational state of the brake system.
The features of the present invention also include alternative or combinational possibilities for the appropriate change-over or selection of one or both brake circuits of the brake system according to the present invention to dynamic operation when the ABS control is activated. The result of this is that a minimum quantity of brake fluid is always kept enclosed in the output pressure chambers of the tandem master cylinder. This minimum quantity of brake fluid can be utilized to provide braking with good deceleration still if the auxiliary pressure source fails.
In practical application of other features of the present invention, the driver can determine from the reaction behavior of the brake pedal whether the ABS control has become active.
The pedal-dependent or piston stroke-dependent change-over of the brake system from static to dynamic operation, in accordance with other aspects of the present invention, provides an additional operational reliability in the case where the ABS control responds in a brake situation in which the coefficients of friction effective between the highway, and the braked vehicle wheels are subjected to drastic fluctuations and the auxiliary pressure supply simultaneously fails.
The present invention also includes means for monitoring the pedal position and the piston positions of suitable displacement sensors, an appropriate processing of the displacement sensors and output signals can be used to achieve important monitoring functions concerning the operational state of the tandem master cylinder of the brake system.
A malfunction of the brake system resulting from a leak in the piston packing provided for sealing the primary piston from working pressure chambers of the tandem master cylinder can be avoided by means of a safety valve included in one embodiment of the present invention.
The present invention also includes working versions of the brake system and its anti-locking system utilizing the return feed principle and viable with low technical effort and advantageous in particular for an integrated design of the brake system and its anti-locking system.
The present invention also includes means for controlling of the change-over of the primary brake circuit to alternating dynamic and static pressure admission provided in Claim 16 for the case where the ABS is operative provides a pulsing of the brake pedal, significant for the operation of the control, as long as the pedal travel covered before activation of the control is not greater than a specified limit.